Switch from brl: to a11y: prefix for metadata properties

[mattgarrish]

In addition to renaming the properties and updating the section on referencing them, I've also removed all the problematic compatibility notes about brl: not being an epub reserved prefix.

Note that this doesn't change the naming of any existing properties or add any new ones. We should tackle those as separate issues.

Fixes #215

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[clapierre]

I think this is fine to start, but some items may need further refinement where just having a11y:xyz won't make sense such as a11y:cellType here I would think a11y:brailleCellType.

[mattgarrish]

Ya, it's easy to get into bikeshedding names with metadata which is why it'd be better to open separate issues for each name that could be changed. I'll reopen #194 as that was already about picking a better name and led to cellType.

[GeorgeKerscher]

I think this looks right, swaping in the A11y prefix.

[mattgarrish]

I found epubcheck doesn't like additional properties while creating the sample, but not sure why. I don't think it verifies any other vocabularies to make sure properties exist.

In any case, that's a minor technicality as we need an updated validator regardless of what we define.